Technical Abstract:
Approximately 10 million tons of oranges are processed in the US each year, producing approximately 5 million tons of citrus peel waste consisting of peel, seeds and segment membranes. Conversion of citrus peel waste into more valuable products, such as fuel ethanol, would greatly benefit the citrus industry. One of the problems in fermenting sugars in citrus processing waste is the presence of peel oil, which predominantly consists of the antimicrobial terpene d-limonene. This study assessed the tolerance of Saccharomyces cerevisiae to limonene during fermentation of hydrolyzed citrus peel waste. Orange peel waste from a commercial facility was hydrolyzed by commercial pectinase, cellulase, and beta-glucosidae enzymes in 250 ml bottles. Peel oil containing 95% limonene was added back to deoiled citrus waste to give test citrus peel waste with limonene concentrations ranging from 0.08-0.43% (v/w). The peel was hydrolyzed, S. cerevisiae added to the bottles, and the bottles rotated at 10-12 rpm for 24 h at 37 °C under anaerobic conditions. Limonene concentration was measured before and after fermentation and ethanol concentration was measured after fermentation. Ethanol production was inhibited when initial limonene concentration was greater than 0.28% and when limonene concentration after fermentation was greater than 0.12%. Between 38% and 60% of the limonene present before fermentation was removed during fermentation.